Engineering Wonders of the World
Volume I

234 ENGINEERING WONDERS OF THE WORLD. Fig. 15.—ELECTRO-PNEUMATIC SIGNALS AT WASHINGTON. Observe that on some American railways a semaphore arm is raised above the horizontal to indicate “clear.” and held off by the power em- ployed. If the track battery’s current fails, or is short circuited by a vehicle being left acciden- tally on the section, or if the signal motor gets out of order, the counter-weight puts the sema- phore to “ danger. ’ In Fig. 16 the train has passed off section 1 ; the current from battery B1 circulates freely through magnet M1, and pulls the signal to “ clear.” As soon as the first pair of train wheels entered sec- tion 2 the current from battery B2 was short cuircuited through these wheels and their axle, the magnet M2 was de-energized, and the semaphore arm of signal 2 was released and went to “ danger.” As long as any wheels of the train are in section 1, that sec- tion is guarded by signal 1 ; but the moment the last pair leaves the section, that signal goes to “ clear.” If for stopped last pair it would, especially at night very inadequately “Overlaps.” any reason the train immediately after the of wheels had passed indirectly, through a power which it switches on, controls the signals. For convenience’ sake we will suppose that the currents from the batteries B, B1, B2 actually work the signals through magnets M1, M2, which attract or release armatures attached by rod work to the semaphore arms. Be it noted that a signal has to be pulled signal time, be protected, as the driver of a train behind might not realize until too late ; therefore the shown in Fig. 16 is inadequate, lines, where no distant or warning but only “ stop ” signals are used, provision is made 2, its presence arrangement On electric Break _<■ ________ _z_ Battery _ B2 Current Section 2 Fig. 16. Signal 1. |M2 (Short Circuited by) ^.BaLtery g ection 1 — B Break I /Clear M'